NVG Presentation Price 2016 In mate choice research population wide preferences for traits that indicate quality are often assumed. However an increasing number of studies indicate that individuals may differ in their preference for a mate. In particular for genetic traits it is expected that individuals might choose a complementary mate rather than one that is universally attractive. In this study we uniquely combined mate preference tests with a field experiment to study the fitness benefits of the measured preferences. We tested 139 wild great tits (Parus major) for their preferences in a six-choice test setup. Both males and females showed an assortative preference for heterozygosity. Moreover, in the subsequent field study, foster pairs with similar heterozygosity levels raised more chicks to fledge than dissimilar foster pairs. Thus by choosing for a partner with similar heterozygosity levels, rather than the absolute level of heterozygosity, individuals gained direct benefits. With these results we’ve shown that individuals differ in their mate preferences depending on their own traits and that finding a compatible mate can increase reproductive success.